A lathe chuck has a chuck body that is held in a lathe headstock and/or tailstock for rotation about an axis. A plurality of radially displaceable jaws on one axial face of the body can be moved between radially outer and inner positions to grip and to release a workpiece or even a tool.
In serial production of large lathed workpieces in a commercial shop, it is standard for the lathe chuck to have an operating member that is displaced at least in part by fluid pressure. Even when the operating member, which usually extends along the lathe axis out the back end of the pedestal it is associated with, is itself actuated from the end of the lathe, it is standard to use a hydraulic or pneumatic boost. The mechanical and normally manual actuation is carried out as the workpiece is mounted in the chuck and in the last stages of dechucking it, and the hydraulic or pneumatic arrangement built inside the chuck body is pressurized or depressurized to increase the force in the desired direction once the workpiece is in proper position.
Obviously the results of a workpiece being brought up to machining speed in a lathe and coming loose from the lathe can be disastrous, leading often to serious injury. Hence it is standard to provide a control system that prevents the lathe drive motor that rotates the headstock about its axis and thereby rotationally entrails the workpiece and tailstock, if provided, from starting unless it receives a signal indicating the chuck is ready or ceases to receive one which indicates that it is unready. The simplest systems simply use a pressure valve in the line to the chucks, so that unless the control system senses pressure in these lines, indicating the chucks are pressurized, it will not be possible to start the lathe drive motor. It is also known to provide on the outside of the chuck an indicator that shows the position of the operating member and to provide even for automatic reading of such indication.
Such systems have not prevented serious accidents. They all have the enormous failing that when the chuck jaws come to their end positions, it is possible for the chuck to be fully pressurized without the jaws securely engaging a workpiece. Another problem is related to the pressure-sensitive system, which occasionally indicates adequate pressurization that is not actually getting into the drive cylinder on the chuck body, but that is blocked somewhat upstream therefrom but downstream from the measuring location.